Motivation

Attempting a mechanics problem from Landau I get a different answer. I wrote up my solution to see if I can spot either where I went wrong, or demonstrate the error, and then posted it to physicsforums. I wasn’t wrong, but the text wasn’t either. Here’s the complete result.

Answer

In the potential the total derivative can be dropped, as can all the constant terms, leaving

so by the above the Lagrangian should be (after also dropping the constant term

This is almost the stated value in the text

We have what appears to be an innocent looking typo (text putting in a instead of a ), but the subsequent text also didn’t make sense. That referred to the omission of the total derivative , which isn’t even a term that I have in my result.

In the physicsforum response it was cleverly pointed out by Dickfore that 1.7 can be recast into a total derivative

The QM I notes updates are strictly cosmetic (the book template is updated to that of classicthesis since it was originally posted). The chapters in QM II are reorganized a bit, grouping things by topic instead by lecture dates.